While I can't speak to the D90 in particular, I've not had a problem mixing and matching cards of different sizes, brands and speeds. Simple answer is your camera's responsiveness will be dependent on the write speed of the slowest card.I started this thread under the D90 hoping for answers specifically to that camera, but I am also interested in what cards are used in other models. For example, with dual slots, is it okay to use cards that have different write speeds? And what if they are different sizes? Is it okay to mix and match? So far I've used identical cards in dual slots as a precaution.
And for the D90, since the recommended size is 8GB, I haven't tried anything larger...again as a precaution.